为什么cout.setf（ios :: fixed）将我的浮点数更改为十六进制？ [英] Why does cout.setf(ios::fixed) change my floats to hexadecimal?

问题描述

我最近遇到了一个与cout.setf（ios :: fixed）有关的怪异问题。我花了相当长的时间来查找原因，以为我想在这里了解更多。

I experienced this weird issue recently having to do with cout.setf(ios::fixed). Took me quite a while to track down the cause and thought I'd ask here to learn more.

问题是这样的-所有浮点数都打印为十六进制数使用cout.setf（ios :: fixed）时。为什么会这样？ ios :: base的文档似乎并不暗示这种情况会发生（至少对我而言）。我正在使用g ++ 5.3.0，下面粘贴是一个最小的示例和输出。

The issue is this - all floating point numbers were printed as hexadecimal numbers when using cout.setf(ios::fixed). Why does this happen? The documentation of ios::base doesn't seem to imply that this will happen (at least to me). I am using g++ 5.3.0 and pasted below is a minimal example and the output.

   #include <iostream>
   #include <complex>

   using namespace std;

   int main(int argc, char const *argv[])
   {
    complex<double> I(0.0, 1.0);
    double pi = M_PI;

    cout.setf(ios::scientific);
    cout<<" I is "<<I<<endl;
    cout<<" Exp(I Pi) "<<exp(I*pi)<<endl;
    cout<<" Cos(Pi) "<<cos(pi)<<endl<<endl;

    cout.setf(ios::fixed);
    cout<<" I is "<<I<<endl;
    cout<<" Exp(I Pi) "<<exp(I*pi)<<endl;
    cout<<" Cos(Pi) "<<cos(pi)<<endl<<endl;

    return 0;
   }

输出

 I is (0.000000e+00,1.000000e+00)
 Exp(I Pi) (-1.000000e+00,1.224647e-16)
 Cos(Pi) -1.000000e+00

 I is (0x0p+0,0x1p+0)
 Exp(I Pi) (-0x1p+0,0x1.1a62633145c07p-53)
 Cos(Pi) -0x1p+0

在此处查看实时示例

请注意，问题已解决当我更改

Note that the issue goes away when I change

 cout.setf(ios::fixed);

到

 cout.setf(ios::fixed, ios::floatfield);

推荐答案

因为您告诉过它。

来自 setf cppreference.com上的文档：

From setf documentation on cppreference.com:

科学-生成浮点类型使用科学计数法或十六进制计数法（如果与固定值结合使用）：请参见 std :: scientific

固定-生成浮点类型使用固定记法，或者十六进制记法（如果与科学结合使用）：请参见 std :: fixed

scientific - generate floating point types using scientific notation, or hex notation if combined with fixed: see std::scientific
fixed - generate floating point types using fixed notation, or hex notation if combined with scientific: see std::fixed

因此，在设置 std :: fixed 时，需要取消设置 std :: scientific （这就是您对 std :: floatfield 的隐藏操作，因为 std :: floatfield 是 std :: scientific | std :: fixed |（std :: scientific | std :: fixed）| 0 ）避免使用十六进制表示法。

So, when setting std::fixed, you need to unset std::scientific (which is what your unmasking of std::floatfield does, because std::floatfield is std::scientific|std::fixed|(std::scientific|std::fixed)|0) to avoid the hex notation.

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